On many occasions tools that are located far underground such as in a wellbore need to be operated at predetermined times from the surface. This is typically accomplished by running small hydraulic lines adjacent a producing tubing string and connecting the lines to one or more operating pistons that are in turn connected to a movable component in the tool. In the case of a downhole isolation valve, for example, the valve can be selectively operated between an open and a closed position from the surface. Options are possible such as for chokes where intermediate positions are also possible between fully open and fully closed.
There are numerous design criteria that affect the design of a control system for a particular application. In most applications space is at a premium in a downhole location and the more space allotted to the control system the less space remains for a through passage in the tool such as a valve. There are also issues of overall tool length as well as a choice between using an annular piston or one or more rod pistons. While certain options address some criteria favorably, they also create issues in other criteria that makes such an option more expensive or in some cases impractical. For example, an annular piston can be used in a downhole valve that can be fairly short and double acting as illustrated in US Publication 2008/0110632. The problem is that the annular piston 26 takes up a lot of space and makes the use of rod piston(s) more practical. Annular pistons experience large seal friction due to the size of the seals that are associated with them. Rod pistons have very small seals and correspondingly less seal friction that has to be overcome with the hydraulic system.
One way to employ rod pistons is in opposed pairs where each bank of rod pistons is single acting. It is important to keep in mind that it is undesirable to have manifolds of control lines on the exterior of a tool housing to reach individual rod piston chambers. What is frequently done is that two housing components are designed to create an annular chamber that communicates with one side of a bank of rod pistons. To enable reverse motion, another bank of rod pistons is oppositely oriented with its own control line connection so that depending on which control line is pressurized, the downhole tool component is urged to move in opposite directions. In an improvement to the design of US Publication 2008/0110632 the annular piston 26 was replaced with pairs of opposed rod pistons as described in U.S. application Ser. No. 12/054,809 filed Mar. 25, 2008. FIG. 1 illustrates a simplified version of the control system used in that application to facilitate understanding of the present invention.
In an application for turning a ball 90 degrees between and open and a closed position, there was shown in that application a shifting slide that engages the turning ball in an offset manner where the ball was pinned for rotation about its center. As shown in FIG. 1, the ball 10 is connected off center at 12 by a shifting slide 14. The ball 10 is also pinned about its center for rotation and that connection is not shown to make the drawing more simple. An annular chamber 16 is formed as threads 18 and 20 are made up to connect components 22 and 24. A control line connection 26 directs fluid pressure into chamber 16 and from there to the individual rod piston bores such as 28 and 30. Pistons such as 32 and 34 each have piston seals such as 36 and 38 such that pressure applied at control line connection 26 will shift all the upper rod pistons such as 32 and 34 in tandem. Since all the pistons such as 32 and 34 are connected to the slide 14, actuating the upper rod pistons turns the ball 10 90 degrees. There is also provided a mirror image array of lower pistons 40 and 42 connected on an opposite end of the slide 14 and is actuated to move in the opposite direction as pistons 32 and 34 when hydraulic pressure is applied at connection 44 that leads into an annular chamber 46 that communicates with all the piston bores such as 48 and 50. Since the pistons 40 and 42 have seals 52 and 54 pressure applied at connection 44 results in opposed ball 10 movement than pressure applied at connection 26.
One issue with this design is that it makes the overall tool length very long because there are duplicated sets of opposed pistons that are single acting, while being disposed end to end for design simplicity and to avoid wasting space that is needed by other components.
The problem with trying to make rod pistons double acting is illustrated in FIG. 2. Solving this problem is one of the objectives of the present invention. Before going into the details of why the FIG. 2 design is a problem it is important to again emphasize that external manifolds of control lines that access every rod piston bore from the outside of the tool housing are frowned upon because they can very easily be bent, damaged or even sheared off when running the tool to the desired position downhole through surrounding tubulars with minimal clearance. To make rod pistons double acting before the present invention was developed, the standard thinking was that there would need to be another housing connection that could define a second annular chamber as shown in FIG. 2. This meant that the piston bore would straddle two threaded components. The problem that arises from such a design is that there is no reliable way to ensure that the portions in a single piston bore in two abutting housing pieces create a second annular chamber that would in fact be in alignment so that the piston would not get into a bind. FIG. 2 illustrates this problem. As before there is an upper annular chamber 16′ made by threads of adjoining housing pieces 22′ and 24′. However, to create a second annular chamber 56 a new housing component 58 will have to be connected to component 22′ at threads 60 and 62. To make such a design work the upper portion of the piston bore 64 in component 22′ would have to perfectly align with the lower portion of the piston bore 66 in component 58. Of course, an additional fixed seal such as 68 and 70 through which a respective rod piston reciprocates would need to be added to each piston as well as an exterior connection for a control line into the chamber 56 to make the system double acting.
Thus the problem solved by the present invention is presented. How can a rod piston design be double acting without requiring another housing connection for an annular chamber that communicates to all the rod piston bores on the other side of a piston seal from control line connection 26′ so as to avoid the issue of piston bore alignment described with regard to FIG. 2. The present invention manages to provide access to the same rod piston bores on the opposite side of piston seals internally without needing another housing component so as to make moot the issue of bore alignment. Those skilled in the art will better understand the invention from the description of the preferred embodiment and the associated drawings while understanding that the full scope of the invention is given by the appended claims. While the preferred embodiment is focused on a downhole isolation valve to illustrate the concept it will be recognized that a wide variety of other tools that are operated with hydraulic control lines can also benefit from the present invention.